When I start the car everything seams normal. I was getting 1100 rpm but after I cleaned out the intake with Seafoam, it came back down to 700.
but every now and then after driving for a bit, it starts to act up again. Even revving up to 3000.

any suggestions to what it can be?

 

Is this a Series III XJ6, fuel injected 4.2 engine?

Cheers
DD

 

yep

 

Does the throttle action feel smooth and easy? Disconnect one end and push-pull the inner cable through the outer case. Should be like silk.

Throttle cable problems are somewhat common on these cars.

Did you set the throttle blade gap to the specified .002" ?

Cheers
DD